NAME

ip — Internet Protocol

SYNOPSIS

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>

int
socket(AF_INET, SOCK_RAW, proto);

DESCRIPTION

IP is the transport layer protocol used by the Internet protocol family. Options may be set at the IP level when using higher-level protocols that are based on IP (such as TCP and UDP). It may also be accessed through a “raw socket” when developing new protocols, or special-purpose applications.

There are several IP-level setsockopt(2) and getsockopt(2) options. IP_OPTIONS may be used to provide IP options to be transmitted in the IP header of each outgoing packet or to examine the header options on incoming packets. IP options may be used with any socket type in the Internet family. The format of IP options to be sent is that specified by the IP protocol specification (RFC-791), with one exception: the list of addresses for Source Route options must include the first-hop gateway at the beginning of the list of gateways. The first-hop gateway address will be extracted from the option list and the size adjusted accordingly before use. To disable previously specified options, use a zero-length buffer:

setsockopt(s, IPPROTO_IP, IP_OPTIONS, NULL, 0);

IP_TOS and IP_TTL may be used to set the type-of-service and time-to-live fields in the IP header for SOCK_STREAM, SOCK_DGRAM, and certain types of SOCK_RAW sockets. For example,

int tos = IPTOS_LOWDELAY;       /* see <netinet/ip.h> */
setsockopt(s, IPPROTO_IP, IP_TOS, &tos, sizeof(tos));

int ttl = 60;                   /* max = 255 */
setsockopt(s, IPPROTO_IP, IP_TTL, &ttl, sizeof(ttl));

IP_MINTTL may be used to set the minimum acceptable TTL a packet must have when received on a socket. All packets with a lower TTL are silently dropped. This option is only really useful when set to 255, preventing packets from outside the directly connected networks reaching local listeners on sockets.

IP_DONTFRAG may be used to set the Don't Fragment flag on IP packets. Currently this option is respected only on udp(4) and raw ip sockets, unless the IP_HDRINCL option has been set. On tcp(4) sockets, the Don't Fragment flag is controlled by the Path MTU Discovery option. Sending a packet larger than the MTU size of the egress interface, determined by the destination address, returns an EMSGSIZE error.

If the IP_ORIGDSTADDR option is enabled on a SOCK_DGRAM socket, the recvmsg(2) call will return the destination IP address and destination port or a UDP datagram. The msg_control field in the msghdr structure points to a buffer that contains a cmsghdr structure followed by the in_sockkaddr structre. The cmsghdr fields have the following values:

cmsg_len = CMSG_LEN(sizeof(struct in_sockaddr))
cmsg_level = IPPROTO_IP
cmsg_type = IP_ORIGDSTADDR

If the IP_RECVDSTADDR option is enabled on a SOCK_DGRAM socket, the recvmsg(2) call will return the destination IP address for a UDP datagram. The msg_control field in the msghdr structure points to a buffer that contains a cmsghdr structure followed by the IP address. The cmsghdr fields have the following values:

cmsg_len = CMSG_LEN(sizeof(struct in_addr))
cmsg_level = IPPROTO_IP
cmsg_type = IP_RECVDSTADDR

The source address to be used for outgoing UDP datagrams on a socket can be specified as ancillary data with a type code of IP_SENDSRCADDR. The msg_control field in the msghdr structure should point to a buffer that contains a cmsghdr structure followed by the IP address. The cmsghdr fields should have the following values:

cmsg_len = CMSG_LEN(sizeof(struct in_addr))
cmsg_level = IPPROTO_IP
cmsg_type = IP_SENDSRCADDR

The socket should be either bound to INADDR_ANY and a local port, and the address supplied with IP_SENDSRCADDR should't be INADDR_ANY, or the socket should be bound to a local address and the address supplied with IP_SENDSRCADDR should be INADDR_ANY. In the latter case bound address is overridden via generic source address selection logic, which would choose IP address of interface closest to destination.

For convenience, IP_SENDSRCADDR is defined to have the same value as IP_RECVDSTADDR, so the IP_RECVDSTADDR control message from recvmsg(2) can be used directly as a control message for sendmsg(2).

If the IP_ONESBCAST option is enabled on a SOCK_DGRAM or a SOCK_RAW socket, the destination address of outgoing broadcast datagrams on that socket will be forced to the undirected broadcast address, INADDR_BROADCAST, before transmission. This is in contrast to the default behavior of the system, which is to transmit undirected broadcasts via the first network interface with the IFF_BROADCAST flag set.

This option allows applications to choose which interface is used to transmit an undirected broadcast datagram. For example, the following code would force an undirected broadcast to be transmitted via the interface configured with the broadcast address 192.168.2.255:

char msg[512];
struct sockaddr_in sin;
int onesbcast = 1;	/* 0 = disable (default), 1 = enable */

setsockopt(s, IPPROTO_IP, IP_ONESBCAST, &onesbcast, sizeof(onesbcast));
sin.sin_addr.s_addr = inet_addr("192.168.2.255");
sin.sin_port = htons(1234);
sendto(s, msg, sizeof(msg), 0, &sin, sizeof(sin));

It is the application's responsibility to set the IP_TTL option to an appropriate value in order to prevent broadcast storms. The application must have sufficient credentials to set the SO_BROADCAST socket level option, otherwise the IP_ONESBCAST option has no effect.

If the IP_BINDANY option is enabled on a SOCK_STREAM, SOCK_DGRAM or a SOCK_RAW socket, one can bind(2) to any address, even one not bound to any available network interface in the system. This functionality (in conjunction with special firewall rules) can be used for implementing a transparent proxy. The PRIV_NETINET_BINDANY privilege is needed to set this option.

If the IP_RECVTTL option is enabled on a SOCK_DGRAM socket, the recvmsg(2) call will return the IP TTL (time to live) field for a UDP datagram. The msg_control field in the msghdr structure points to a buffer that contains a cmsghdr structure followed by the TTL. The cmsghdr fields have the following values:

cmsg_len = CMSG_LEN(sizeof(u_char))
cmsg_level = IPPROTO_IP
cmsg_type = IP_RECVTTL

If the IP_RECVTOS option is enabled on a SOCK_DGRAM socket, the recvmsg(2) call will return the IP TOS (type of service) field for a UDP datagram. The msg_control field in the msghdr structure points to a buffer that contains a cmsghdr structure followed by the TOS. The cmsghdr fields have the following values:

cmsg_len = CMSG_LEN(sizeof(u_char))
cmsg_level = IPPROTO_IP
cmsg_type = IP_RECVTOS

If the IP_RECVIF option is enabled on a SOCK_DGRAM socket, the recvmsg(2) call returns a struct sockaddr_dl corresponding to the interface on which the packet was received. The msg_control field in the msghdr structure points to a buffer that contains a cmsghdr structure followed by the struct sockaddr_dl. The cmsghdr fields have the following values:

cmsg_len = CMSG_LEN(sizeof(struct sockaddr_dl))
cmsg_level = IPPROTO_IP
cmsg_type = IP_RECVIF

IP_PORTRANGE may be used to set the port range used for selecting a local port number on a socket with an unspecified (zero) port number. It has the following possible values:

IP_PORTRANGE_DEFAULT

use the default range of values, normally IPPORT_HIFIRSTAUTO through IPPORT_HILASTAUTO. This is adjustable through the sysctl setting: net.inet.ip.portrange.first and net.inet.ip.portrange.last.

IP_PORTRANGE_HIGH

use a high range of values, normally IPPORT_HIFIRSTAUTO and IPPORT_HILASTAUTO. This is adjustable through the sysctl setting: net.inet.ip.portrange.hifirst and net.inet.ip.portrange.hilast.

IP_PORTRANGE_LOW

use a low range of ports, which are normally restricted to privileged processes on UNIX systems. The range is normally from IPPORT_RESERVED - 1 down to IPPORT_RESERVEDSTART in descending order. This is adjustable through the sysctl setting: net.inet.ip.portrange.lowfirst and net.inet.ip.portrange.lowlast.

The range of privileged ports which only may be opened by root-owned processes may be modified by the net.inet.ip.portrange.reservedlow and net.inet.ip.portrange.reservedhigh sysctl settings. The values default to the traditional range, 0 through IPPORT_RESERVED - 1 (0 through 1023), respectively. Note that these settings do not affect and are not accounted for in the use or calculation of the other net.inet.ip.portrange values above. Changing these values departs from UNIX tradition and has security consequences that the administrator should carefully evaluate before modifying these settings.

Ports are allocated at random within the specified port range in order to increase the difficulty of random spoofing attacks. In scenarios such as benchmarking, this behavior may be undesirable. In these cases, net.inet.ip.portrange.randomized can be used to toggle randomization off. If more than net.inet.ip.portrange.randomcps ports have been allocated in the last second, then return to sequential port allocation. Return to random allocation only once the current port allocation rate drops below net.inet.ip.portrange.randomcps for at least net.inet.ip.portrange.randomtime seconds. The default values for net.inet.ip.portrange.randomcps and net.inet.ip.portrange.randomtime are 10 port allocations per second and 45 seconds correspondingly.

u_char ttl;	/* range: 0 to 255, default = 1 */
setsockopt(s, IPPROTO_IP, IP_MULTICAST_TTL, &ttl, sizeof(ttl));

struct in_addr addr;
setsockopt(s, IPPROTO_IP, IP_MULTICAST_IF, &addr, sizeof(addr));

u_char loop;	/* 0 = disable, 1 = enable (default) */
setsockopt(s, IPPROTO_IP, IP_MULTICAST_LOOP, &loop, sizeof(loop));

struct ip_mreq mreq;
setsockopt(s, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, sizeof(mreq));

struct ip_mreq {
    struct in_addr imr_multiaddr; /* IP multicast address of group */
    struct in_addr imr_interface; /* local IP address of interface */
}

struct ip_mreq mreq;
setsockopt(s, IPPROTO_IP, IP_DROP_MEMBERSHIP, &mreq, sizeof(mreq));

struct ip_mreq_source mreqs;
setsockopt(s, IPPROTO_IP, IP_BLOCK_SOURCE, &mreqs, sizeof(mreqs));

struct ip_mreq_source {
    struct in_addr imr_multiaddr; /* IP multicast address of group */
    struct in_addr imr_sourceaddr; /* IP address of source */
    struct in_addr imr_interface; /* local IP address of interface */
}

struct ip_mreq_source mreqs;
setsockopt(s, IPPROTO_IP, IP_UNBLOCK_SOURCE, &mreqs, sizeof(mreqs));

struct ip_mreq_source mreqs;
setsockopt(s, IPPROTO_IP, IP_ADD_SOURCE_MEMBERSHIP, &mreqs, sizeof(mreqs));

struct ip_mreq_source mreqs;
setsockopt(s, IPPROTO_IP, IP_DROP_SOURCE_MEMBERSHIP, &mreqs, sizeof(mreqs));

#include <netinet/in_systm.h>
#include <netinet/ip.h>

int hincl = 1;                  /* 1 = on, 0 = off */
setsockopt(s, IPPROTO_IP, IP_HDRINCL, &hincl, sizeof(hincl));

ip->ip_v = IPVERSION;
ip->ip_hl = hlen >> 2;
ip->ip_id = 0;  /* 0 means kernel set appropriate value */
ip->ip_off = htons(offset);
ip->ip_len = htons(len);

ERRORS

A socket operation may fail with one of the following errors returned:

[EISCONN]

when trying to establish a connection on a socket which already has one, or when trying to send a datagram with the destination address specified and the socket is already connected;

[ENOTCONN]

when trying to send a datagram, but no destination address is specified, and the socket has not been connected;

[ENOBUFS]

when the system runs out of memory for an internal data structure;

[EADDRNOTAVAIL]

when an attempt is made to create a socket with a network address for which no network interface exists.

[EACCES]

when an attempt is made to create a raw IP socket by a non-privileged process.

The following errors specific to IP may occur when setting or getting IP options:

[EINVAL]

An unknown socket option name was given.

[EINVAL]

The IP option field was improperly formed; an option field was shorter than the minimum value or longer than the option buffer provided.

The following errors may occur when attempting to send IP datagrams via a “raw socket” with the IP_HDRINCL option set:

[EINVAL]

The user-supplied ip_len field was not equal to the length of the datagram written to the socket.

SEE ALSO

getsockopt(2), recv(2), send(2), byteorder(3), CMSG_DATA(3), sourcefilter(3), icmp(4), igmp(4), inet(4), intro(4), multicast(4)

D. Thaler, B. Fenner, and B. Quinn, Socket Interface Extensions for Multicast Source Filters, RFC 3678, Jan 2004.

HISTORY

The ip protocol appeared in 4.2BSD. The ip_mreqn structure appeared in Linux 2.4.

BUGS

Before FreeBSD 10.0 packets received on raw IP sockets had the ip_hl subtracted from the ip_len field.

Before FreeBSD 11.0 packets received on raw IP sockets had the ip_len and ip_off fields converted to host byte order. Packets written to raw IP sockets were expected to have ip_len and ip_off in host byte order.